Linux implements the Internet Protocol, version 4, described in RFC 791
and RFC 1122. ip contains a level 2 multicasting implementation
conforming to RFC 1112. It also contains an IP router including a
packet filter.

The programming interface is BSD-sockets compatible. For more
information on sockets, see socket(7).

Valid socket types are SOCK_STREAM to open a tcp(7)
socket, SOCK_DGRAM to open a udp(7) socket, or SOCK_RAW
to open a raw(7) socket to access the IP protocol directly.
protocol is the IP protocol in the IP header to be received or sent.
The only valid values for protocol are 0 and IPPROTO_TCP for
TCP sockets, and 0 and IPPROTO_UDP for UDP sockets. For
SOCK_RAW you may specify a valid IANA IP protocol defined in
RFC 1700 assigned numbers.

When a process wants to receive new incoming packets or
connections, it should bind a socket to a local interface address using
bind(2). In this case, only one IP socket may be bound to any given
local (address, port) pair. When INADDR_ANY is specified in the bind
call, the socket will be bound to all local interfaces. When
listen(2) is called on an unbound socket, the socket is automatically
bound to a random free port with the local address set to INADDR_ANY.
When connect(2) is called on an unbound socket, the socket is
automatically bound to a random free port or to a usable shared port with
the local address set to INADDR_ANY.

A TCP local socket address that has been bound is unavailable for
some time after closing, unless the SO_REUSEADDR flag has been set.
Care should be taken when using this flag as it makes TCP less reliable.

An IP socket address is defined as a combination of an IP interface address and
a 16-bit port number. The basic IP protocol does not supply port numbers, they
are implemented by higher level protocols like udp(7) and
tcp(7). On raw sockets sin_port is set to the IP protocol.

sin_family is always set to AF_INET. This is
required; in Linux 2.2 most networking functions return EINVAL when
this setting is missing. sin_port contains the port in network byte
order. The port numbers below 1024 are called privileged ports (or
sometimes: reserved ports). Only a privileged process (on Linux: a
process that has the CAP_NET_BIND_SERVICE capability in the user
namespace governing its network namespace) may bind(2) to these
sockets. Note that the raw IPv4 protocol as such has no concept of a port,
they are implemented only by higher protocols like tcp(7) and
udp(7).

sin_addr is the IP host address. The s_addr member
of struct in_addr contains the host interface address in network byte
order. in_addr should be assigned one of the INADDR_* values
(e.g., INADDR_LOOPBACK) using htonl(3) or set using the
inet_aton(3), inet_addr(3), inet_makeaddr(3) library
functions or directly with the name resolver (see
gethostbyname(3)).

IPv4 addresses are divided into unicast, broadcast, and multicast
addresses. Unicast addresses specify a single interface of a host, broadcast
addresses specify all hosts on a network, and multicast addresses address
all hosts in a multicast group. Datagrams to broadcast addresses can be sent
or received only when the SO_BROADCAST socket flag is set. In the
current implementation, connection-oriented sockets are allowed to use only
unicast addresses.

Note that the address and the port are always stored in network
byte order. In particular, this means that you need to call htons(3)
on the number that is assigned to a port. All address/port manipulation
functions in the standard library work in network byte order.

There are several special addresses: INADDR_LOOPBACK
(127.0.0.1) always refers to the local host via the loopback device;
INADDR_ANY (0.0.0.0) means any address for binding;
INADDR_BROADCAST (255.255.255.255) means any host and has the same
effect on bind as INADDR_ANY for historical reasons.

IP supports some protocol-specific socket options that can be set with
setsockopt(2) and read with getsockopt(2). The socket option
level for IP is IPPROTO_IP. A boolean integer flag is zero when it is
false, otherwise true.

imr_multiaddr contains the address of the multicast group
the application wants to join or leave. It must be a valid multicast address
(or setsockopt(2) fails with the error EINVAL).
imr_address is the address of the local interface with which the
system should join the multicast group; if it is equal to INADDR_ANY,
an appropriate interface is chosen by the system. imr_ifindex is the
interface index of the interface that should join/leave the
imr_multiaddr group, or 0 to indicate any interface.

The ip_mreqn structure is available only since Linux 2.2. For
compatibility, the old ip_mreq structure (present since Linux 1.2)
is still supported; it differs from ip_mreqn only by not including
the imr_ifindex field. (The kernel determines which structure is
being passed based on the size passed in optlen.)

The ip_mreq_source structure is similar to ip_mreqn
described under IP_ADD_MEMBERSIP. The imr_multiaddr field
contains the address of the multicast group the application wants to join or
leave. The imr_interface field is the address of the local interface
with which the system should join the multicast group. Finally, the
imr_sourceaddr field contains the address of the source the
application wants to receive data from.

This option can be used multiple times to allow receiving data from more
than one source.

IP_BIND_ADDRESS_NO_PORT (since Linux 4.2)

Inform the kernel to not reserve an ephemeral port when using
bind(2) with a port number of 0. The port will later be
automatically chosen at connect(2) time, in a way that allows
sharing a source port as long as the 4-tuple is unique.

IP_BLOCK_SOURCE (since Linux 2.4.22 / 2.5.68)

Stop receiving multicast data from a specific source in a given group.
This is valid only after the application has subscribed to the multicast
group using either IP_ADD_MEMBERSHIP or
IP_ADD_SOURCE_MEMBERSHIP.

Argument is an ip_mreq_source structure as described under
IP_ADD_SOURCE_MEMBERSHIP.

IP_DROP_MEMBERSHIP (since Linux 1.2)

Leave a multicast group. Argument is an ip_mreqn or ip_mreq
structure similar to IP_ADD_MEMBERSHIP.

IP_DROP_SOURCE_MEMBERSHIP (since Linux 2.4.22 / 2.5.68)

Leave a source-specific group—that is, stop receiving data from a
given multicast group that come from a given source. If the application
has subscribed to multiple sources within the same group, data from the
remaining sources will still be delivered. To stop receiving data from all
sources at once, use IP_DROP_MEMBERSHIP.

Argument is an ip_mreq_source structure as described under
IP_ADD_SOURCE_MEMBERSHIP.

IP_FREEBIND (since Linux 2.4)

If enabled, this boolean option allows binding to an IP address that is
nonlocal or does not (yet) exist. This permits listening on a socket,
without requiring the underlying network interface or the specified
dynamic IP address to be up at the time that the application is trying to
bind to it. This option is the per-socket equivalent of the
ip_nonlocal_bind/proc interface described below.

IP_HDRINCL (since Linux 2.0)

If enabled, the user supplies an IP header in front of the user data.
Valid only for SOCK_RAW sockets; see raw(7) for more
information. When this flag is enabled, the values set by
IP_OPTIONS, IP_TTL, and IP_TOS are ignored.

IP_MSFILTER (since Linux 2.4.22 / 2.5.68)

This option provides access to the advanced full-state filtering API.
Argument is an ip_msfilter structure.

There are two macros, MCAST_INCLUDE and
MCAST_EXCLUDE, which can be used to specify the filtering mode.
Additionally, the IP_MSFILTER_SIZE(n) macro exists to determine how
much memory is needed to store ip_msfilter structure with n
sources in the source list.

For the full description of multicast source filtering refer to RFC
3376.

IP_MTU (since Linux 2.2)

Retrieve the current known path MTU of the current socket. Returns an
integer.

IP_MTU is valid only for getsockopt(2) and can be employed
only when the socket has been connected.

IP_MTU_DISCOVER (since Linux 2.2)

Set or receive the Path MTU Discovery setting for a socket. When enabled,
Linux will perform Path MTU Discovery as defined in RFC 1191 on
SOCK_STREAM sockets. For non-SOCK_STREAM sockets,
IP_PMTUDISC_DO forces the don't-fragment flag to be set on all
outgoing packets. It is the user's responsibility to packetize the data in
MTU-sized chunks and to do the retransmits if necessary. The kernel will
reject (with EMSGSIZE) datagrams that are bigger than the known
path MTU. IP_PMTUDISC_WANT will fragment a datagram if needed
according to the path MTU, or will set the don't-fragment flag
otherwise.

The system-wide default can be toggled between IP_PMTUDISC_WANT and
IP_PMTUDISC_DONT by writing (respectively, zero and nonzero values)
to the /proc/sys/net/ipv4/ip_no_pmtu_disc file.

Path MTU discovery value

Meaning

IP_PMTUDISC_WANT

Use per-route settings.

IP_PMTUDISC_DONT

Never do Path MTU Discovery.

IP_PMTUDISC_DO

Always do Path MTU Discovery.

IP_PMTUDISC_PROBE

Set DF but ignore Path MTU.

When PMTU discovery is enabled, the kernel automatically keeps
track of the path MTU per destination host. When it is connected to a
specific peer with connect(2), the currently known path MTU can
be retrieved conveniently using the IP_MTU socket option (e.g.,
after an EMSGSIZE error occurred). The path MTU may change over
time. For connectionless sockets with many destinations, the new MTU for
a given destination can also be accessed using the error queue (see
IP_RECVERR). A new error will be queued for every incoming MTU
update.

While MTU discovery is in progress, initial packets from datagram sockets
may be dropped. Applications using UDP should be aware of this and not
take it into account for their packet retransmit strategy.

To bootstrap the path MTU discovery process on unconnected sockets, it is
possible to start with a big datagram size (headers up to 64 kilobytes
long) and let it shrink by updates of the path MTU.

To get an initial estimate of the path MTU, connect a datagram socket to
the destination address using connect(2) and retrieve the MTU by
calling getsockopt(2) with the IP_MTU option.

It is possible to implement RFC 4821 MTU probing with SOCK_DGRAM or
SOCK_RAW sockets by setting a value of IP_PMTUDISC_PROBE
(available since Linux 2.6.22). This is also particularly useful for
diagnostic tools such as tracepath(8) that wish to deliberately
send probe packets larger than the observed Path MTU.

IP_MULTICAST_ALL (since Linux 2.6.31)

This option can be used to modify the delivery policy of multicast
messages to sockets bound to the wildcard INADDR_ANY address. The
argument is a boolean integer (defaults to 1). If set to 1, the socket
will receive messages from all the groups that have been joined globally
on the whole system. Otherwise, it will deliver messages only from the
groups that have been explicitly joined (for example via the
IP_ADD_MEMBERSHIP option) on this particular socket.

IP_MULTICAST_IF (since Linux 1.2)

Set the local device for a multicast socket. The argument for
setsockopt(2) is an ip_mreqn or (since Linux 3.5)
ip_mreq structure similar to IP_ADD_MEMBERSHIP, or an
in_addr structure. (The kernel determines which structure is being
passed based on the size passed in optlen.) For
getsockopt(2), the argument is an in_addr structure.

IP_MULTICAST_LOOP (since Linux 1.2)

Set or read a boolean integer argument that determines whether sent
multicast packets should be looped back to the local sockets.

IP_MULTICAST_TTL (since Linux 1.2)

Set or read the time-to-live value of outgoing multicast packets for this
socket. It is very important for multicast packets to set the smallest TTL
possible. The default is 1 which means that multicast packets don't leave
the local network unless the user program explicitly requests it. Argument
is an integer.

IP_NODEFRAG (since Linux 2.6.36)

If enabled (argument is nonzero), the reassembly of outgoing packets is
disabled in the netfilter layer. The argument is an integer.

This option is valid only for SOCK_RAW sockets.

IP_OPTIONS (since Linux 2.0)

Set or get the IP options to be sent with every packet from this socket.
The arguments are a pointer to a memory buffer containing the options and
the option length. The setsockopt(2) call sets the IP options
associated with a socket. The maximum option size for IPv4 is 40 bytes.
See RFC 791 for the allowed options. When the initial connection
request packet for a SOCK_STREAM socket contains IP options, the IP
options will be set automatically to the options from the initial packet
with routing headers reversed. Incoming packets are not allowed to change
options after the connection is established. The processing of all
incoming source routing options is disabled by default and can be enabled
by using the accept_source_route/proc interface. Other
options like timestamps are still handled. For datagram sockets, IP
options can be only set by the local user. Calling getsockopt(2)
with IP_OPTIONS puts the current IP options used for sending into
the supplied buffer.

IP_PKTINFO (since Linux 2.2)

Pass an IP_PKTINFO ancillary message that contains a pktinfo
structure that supplies some information about the incoming packet. This
only works for datagram oriented sockets. The argument is a flag that
tells the socket whether the IP_PKTINFO message should be passed or
not. The message itself can only be sent/retrieved as control message with
a packet using recvmsg(2) or sendmsg(2).

ipi_ifindex is the unique index of the interface the packet was
received on. ipi_spec_dst is the local address of the packet and
ipi_addr is the destination address in the packet header. If
IP_PKTINFO is passed to sendmsg(2) and ipi_spec_dst
is not zero, then it is used as the local source address for the routing
table lookup and for setting up IP source route options. When
ipi_ifindex is not zero, the primary local address of the interface
specified by the index overwrites ipi_spec_dst for the routing
table lookup.

IP_RECVERR (since Linux 2.2)

Enable extended reliable error message passing. When enabled on a datagram
socket, all generated errors will be queued in a per-socket error queue.
When the user receives an error from a socket operation, the errors can be
received by calling recvmsg(2) with the MSG_ERRQUEUE flag
set. The sock_extended_err structure describing the error will be
passed in an ancillary message with the type IP_RECVERR and the
level IPPROTO_IP. This is useful for reliable error handling on
unconnected sockets. The received data portion of the error queue contains
the error packet.

The IP_RECVERR control message contains a sock_extended_err
structure:

ee_errno contains the errno number of the queued error.
ee_origin is the origin code of where the error originated. The
other fields are protocol-specific. The macro SO_EE_OFFENDER
returns a pointer to the address of the network object where the error
originated from given a pointer to the ancillary message. If this address
is not known, the sa_family member of the sockaddr contains
AF_UNSPEC and the other fields of the sockaddr are
undefined.

IP uses the sock_extended_err structure as follows:
ee_origin is set to SO_EE_ORIGIN_ICMP for errors received as
an ICMP packet, or SO_EE_ORIGIN_LOCAL for locally generated errors.
Unknown values should be ignored. ee_type and ee_code are
set from the type and code fields of the ICMP header. ee_info
contains the discovered MTU for EMSGSIZE errors. The message also
contains the sockaddr_in of the node caused the error, which can be
accessed with the SO_EE_OFFENDER macro. The sin_family field
of the SO_EE_OFFENDER address is AF_UNSPEC when the source
was unknown. When the error originated from the network, all IP options
(IP_OPTIONS, IP_TTL, etc.) enabled on the socket and
contained in the error packet are passed as control messages. The payload
of the packet causing the error is returned as normal payload. Note that
TCP has no error queue; MSG_ERRQUEUE is not permitted on
SOCK_STREAM sockets. IP_RECVERR is valid for TCP, but all
errors are returned by socket function return or SO_ERROR
only.

For raw sockets, IP_RECVERR enables passing of all received ICMP
errors to the application, otherwise errors are only reported on connected
sockets

It sets or retrieves an integer boolean flag. IP_RECVERR defaults
to off.

IP_RECVOPTS (since Linux 2.2)

Pass all incoming IP options to the user in a IP_OPTIONS control
message. The routing header and other options are already filled in for
the local host. Not supported for SOCK_STREAM sockets.

IP_RECVORIGDSTADDR (since Linux 2.6.29)

This boolean option enables the IP_ORIGDSTADDR ancillary message in
recvmsg(2), in which the kernel returns the original destination
address of the datagram being received. The ancillary message contains a
struct sockaddr_in.

IP_RECVTOS (since Linux 2.2)

If enabled, the IP_TOS ancillary message is passed with incoming
packets. It contains a byte which specifies the Type of Service/Precedence
field of the packet header. Expects a boolean integer flag.

IP_RECVTTL (since Linux 2.2)

When this flag is set, pass a IP_TTL control message with the
time-to-live field of the received packet as a 32 bit integer. Not
supported for SOCK_STREAM sockets.

IP_RETOPTS (since Linux 2.2)

Identical to IP_RECVOPTS, but returns raw unprocessed options with
timestamp and route record options not filled in for this hop.

IP_ROUTER_ALERT (since Linux 2.2)

Pass all to-be forwarded packets with the IP Router Alert option set to
this socket. Valid only for raw sockets. This is useful, for instance, for
user-space RSVP daemons. The tapped packets are not forwarded by the
kernel; it is the user's responsibility to send them out again. Socket
binding is ignored, such packets are only filtered by protocol. Expects an
integer flag.

IP_TOS (since Linux 1.0)

Set or receive the Type-Of-Service (TOS) field that is sent with every IP
packet originating from this socket. It is used to prioritize packets on
the network. TOS is a byte. There are some standard TOS flags defined:
IPTOS_LOWDELAY to minimize delays for interactive traffic,
IPTOS_THROUGHPUT to optimize throughput, IPTOS_RELIABILITY
to optimize for reliability, IPTOS_MINCOST should be used for
"filler data" where slow transmission doesn't matter. At most
one of these TOS values can be specified. Other bits are invalid and shall
be cleared. Linux sends IPTOS_LOWDELAY datagrams first by default,
but the exact behavior depends on the configured queueing discipline. Some
high-priority levels may require superuser privileges (the
CAP_NET_ADMIN capability).

IP_TRANSPARENT (since Linux 2.6.24)

Setting this boolean option enables transparent proxying on this socket.
This socket option allows the calling application to bind to a nonlocal IP
address and operate both as a client and a server with the foreign address
as the local endpoint. NOTE: this requires that routing be set up in a way
that packets going to the foreign address are routed through the TProxy
box (i.e., the system hosting the application that employs the
IP_TRANSPARENT socket option). Enabling this socket option requires
superuser privileges (the CAP_NET_ADMIN capability).

TProxy redirection with the iptables TPROXY target also requires that this
option be set on the redirected socket.

IP_TTL (since Linux 1.0)

Set or retrieve the current time-to-live field that is used in every
packet sent from this socket.

IP_UNBLOCK_SOURCE (since Linux 2.4.22 / 2.5.68)

Unblock previously blocked multicast source. Returns EADDRNOTAVAIL
when given source is not being blocked.

Argument is an ip_mreq_source structure as described under
IP_ADD_SOURCE_MEMBERSHIP.

The IP protocol supports a set of /proc interfaces to configure some
global parameters. The parameters can be accessed by reading or writing files
in the directory /proc/sys/net/ipv4/. Interfaces described as
Boolean take an integer value, with a nonzero value ("true")
meaning that the corresponding option is enabled, and a zero value
("false") meaning that the option is disabled.

ip_always_defrag (Boolean; since Linux 2.2.13)

[New with kernel 2.2.13; in earlier kernel versions this feature was
controlled at compile time by the CONFIG_IP_ALWAYS_DEFRAG option;
this option is not present in 2.4.x and later]

When this boolean flag is enabled (not equal 0), incoming fragments (parts
of IP packets that arose when some host between origin and destination
decided that the packets were too large and cut them into pieces) will be
reassembled (defragmented) before being processed, even if they are about
to be forwarded.

Enable only if running either a firewall that is the sole link to your
network or a transparent proxy; never ever use it for a normal router or
host. Otherwise, fragmented communication can be disturbed if the
fragments travel over different links. Defragmentation also has a large
memory and CPU time cost.

This is automagically turned on when masquerading or transparent proxying
are configured.

ip_autoconfig (since Linux 2.2 to 2.6.17)

Not documented.

ip_default_ttl (integer; default: 64; since Linux 2.2)

Set the default time-to-live value of outgoing packets. This can be
changed per socket with the IP_TTL option.

Enable IP forwarding with a boolean flag. IP forwarding can be also set on
a per-interface basis.

ip_local_port_range (since Linux 2.2)

This file contains two integers that define the default local port range
allocated to sockets that are not explicitly bound to a port
number—that is, the range used for ephemeral ports. An
ephemeral port is allocated to a socket in the following
circumstances:

the port number in a socket address is specified as 0 when calling
bind(2);

sendto(2) is called on a datagram socket that was not previously
bound.

Allocation of ephemeral ports starts with the first number in
ip_local_port_range and ends with the second number. If the range
of ephemeral ports is exhausted, then the relevant system call returns an
error (but see BUGS).

Note that the port range in ip_local_port_range should not conflict
with the ports used by masquerading (although the case is handled). Also,
arbitrary choices may cause problems with some firewall packet filters
that make assumptions about the local ports in use. The first number
should be at least greater than 1024, or better, greater than 4096, to
avoid clashes with well known ports and to minimize firewall
problems.

ip_no_pmtu_disc (Boolean; default: disabled; since Linux 2.2)

If enabled, don't do Path MTU Discovery for TCP sockets by default. Path
MTU discovery may fail if misconfigured firewalls (that drop all ICMP
packets) or misconfigured interfaces (e.g., a point-to-point link where
the both ends don't agree on the MTU) are on the path. It is better to fix
the broken routers on the path than to turn off Path MTU Discovery
globally, because not doing it incurs a high cost to the network.

ip_nonlocal_bind (Boolean; default: disabled; since Linux 2.4)

If set, allows processes to bind(2) to nonlocal IP addresses, which
can be quite useful, but may break some applications.

The user tried to execute an operation without the necessary permissions.
These include: sending a packet to a broadcast address without having the
SO_BROADCAST flag set; sending a packet via a prohibit
route; modifying firewall settings without superuser privileges (the
CAP_NET_ADMIN capability); binding to a privileged port without
superuser privileges (the CAP_NET_BIND_SERVICE capability).

EADDRINUSE

Tried to bind to an address already in use.

EADDRNOTAVAIL

A nonexistent interface was requested or the requested source address was
not local.

EAGAIN

Operation on a nonblocking socket would block.

EALREADY

A connection operation on a nonblocking socket is already in
progress.

Be very careful with the SO_BROADCAST option - it is not
privileged in Linux. It is easy to overload the network with careless
broadcasts. For new application protocols it is better to use a multicast
group instead of broadcasting. Broadcasting is discouraged.

Some other BSD sockets implementations provide
IP_RCVDSTADDR and IP_RECVIF socket options to get the
destination address and the interface of received datagrams. Linux has the
more general IP_PKTINFO for the same task.

Some BSD sockets implementations also provide an IP_RECVTTL
option, but an ancillary message with type IP_RECVTTL is passed with
the incoming packet. This is different from the IP_TTL option used in
Linux.

For compatibility with Linux 2.0, the obsolete socket(AF_INET, SOCK_PACKET,
protocol) syntax is still supported to open a
packet(7) socket. This is deprecated and should be replaced by
socket(AF_PACKET, SOCK_RAW, protocol) instead. The main
difference is the new sockaddr_ll address structure for generic link
layer information instead of the old sockaddr_pkt.

This page is part of release 5.00 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the latest
version of this page, can be found at https://www.kernel.org/doc/man-pages/.